1. Field of the Invention
The invention relates to the surgical stabilisation of the spine.
2. The Prior Art
The principle of stabilisation of algesic spine by means of vertebral arthrodesis has been known to orthopaedic-surgeons for many years. Arthrodesis is achieved after perfect immobilisation of the vertebrae by means of a rigid frame which acts as a support to the pathological spine. This frame is constituted of plates, or non distorsion bars, placed in situ, firmly fixed to the spine using screws or hooks.
If a bar is used, these screws include a threaded rod which is intended to be set into a vertebra, and an extra-osseous head tulip-shaped and slotted to the diameter of the said bar. The bar is bolted to the screw by means of a point screw or a vice. However, no matter how tight the point screw or the vice, the permanent stress which the spine undergoes finally causes the bar to shift in its support.
To limit this drawback, special surface structures of the bar have been suggested (serrated surface structure, cross-hatched surface structure EP 0348 272), or the use of special locking rings has been advocated (FR 83 07450/2545350). Nonetheless, about 7% of failures occur, due to the shifting of the bars in their fixing members.